Impact of hypertension-related avoidable hospitalization on all-cause mortality in older patients with hypertension: a nationwide retrospective cohort study in Korea

Data and study population

In Korea, the National Health Insurance Service (NHIS) operates as a single-payer system and has provided universal health coverage since 1989 [20]. Approximately 97% of the population is covered by NHIS health insurance, while the remaining 3% are supported by a Medical Aid program [21]. Korean citizens pay monthly health insurance premiums based on their income and assets; in return, the NHIS covers most healthcare costs, excluding out-of-pocket expenses [22,23]. Because healthcare providers bill the NHIS directly for services, the NHIS maintains comprehensive healthcare utilization data for the entire population. Consequently, NHIS data are considered nationally representative.

This study utilized data from the NHIS-Senior Cohort covering the years 2008 to 2019. The NHIS-Senior Cohort is a stratified random sample representing approximately 8% of older adults aged 60-80 years in 2008 and was further categorized by age, sex, health insurance premiums, and region [24]. The study population comprised older patients aged ≥60 years who were first diagnosed with hypertension (International Classification of Diseases, 10th revision [ICD-10] code: I10) as the principal diagnosis between 2008 and 2014 (n=113,195). Recognizing that sole reliance on ICD-10 codes might include non-hypertensive patients due to inherent discrepancies in claims data, we further restricted inclusion to individuals who were prescribed antihypertensive medications within 1 year of their initial hypertension diagnosis (n=99,401) and who had at least 4 outpatient visits (n=67,377). The list of antihypertensive drugs used to define the inclusion criteria is presented in Supplementary Material 1. Individuals hospitalized for hypertension at the time of initial diagnosis (n=1,231) and those who died within 1 year of diagnosis (n=460) were excluded. The final study sample comprised 65,686 older patients with hypertension (Figure 1).

Figure 1.

Data flow chart.

Variables

Statistical analysis

The analyses in this study were conducted in 3 stages. First, a chi-squared test was employed to evaluate the relationship between the general characteristics of the study population and both short-term and long-term all-cause mortality, with a p-value threshold of <0.05 indicating statistical significance. Second, survival analysis was performed to examine the association between hypertension-related avoidable hospitalizations and both short-term and long-term all-cause mortality. The proportional hazards assumption was verified using Kaplan-Meier survival curves, and the statistical significance of these curves was assessed with the log-rank test (p<0.05). Cox proportional hazards models were then used for regression analysis, with statistical significance determined when the 95% confidence interval (CI) did not include 1. All survival analyses—including Kaplan-Meier curves and number-at-risk tables—were based on the time to the first occurrence of a hypertension-related avoidable hospitalization; recurrent events were not considered. Third, sensitivity analyses were conducted using 2 approaches—subgroup analysis and propensity score matching (PSM)—to reinforce the robustness of our findings and reduce selection bias. First, PSM, a quasi-experimental method that approximates randomized clinical trials in observational studies, was applied to ensure greater comparability between case and control groups [28]. We employed the greedy matching method with a 1:3 ratio, and the adequacy of matching was assessed using the standardized mean difference, with an absolute threshold of <0.1. Following PSM, a Cox regression analysis was conducted on the matched cohort. Second, subgroup analyses were performed to explore potential disparities in the association between hypertension-related avoidable hospitalizations and all-cause mortality across different socioeconomic and regional subgroups. Additionally, a subgroup analysis based on MPR adherence was conducted to evaluate the effect of proper hypertension management. Prior to these analyses, a joint test was used to assess the interaction effect with hypertension-related avoidable hospitalization, with statistical significance defined at a p-value threshold of <0.05. All regression analyses were adjusted for the covariates included in the study. Given that hypertension-related avoidable hospitalizations are time-dependent variables that may recur, the landmark method—a robust approach for controlling immortal time bias—was applied [29]. Because the outcome measure was all-cause mortality, patients experiencing hypertension-related avoidable hospitalizations inherently had an immortal time period from the time of hypertension diagnosis until the hospitalization event, during which death could not occur. To address this bias, a landmark period of 1 year from the date of hypertension diagnosis was established. Under this method, only the exposure status within the landmark period was considered, and any exposures occurring afterward were disregarded [29]. Consequently, the observation window for hypertension-related avoidable hospitalizations was defined as the 1-year landmark period, and individuals who experienced the outcome event during this period were excluded. All analyses were performed using SAS version 9.4 (SAS Institute Inc., Cary, NC, USA) and R version 4.3.0 (R Foundation for Statistical Computing, Vienna, Austria).

Ethics statement

The Institutional Review Board of the National Health Insurance Service Ilsan Hospital, Korea waived ethical approval for this study (NHIMC 2023-06-022) because only secondary data containing anonymized and encrypted personal information were used.

Key findings

This study investigated the association between hypertension-related avoidable hospitalizations and both short-term and long-term all-cause mortality in older patients with hypertension. The analysis revealed that older patients who experienced such hospitalizations had an increased risk of mortality in both the short-term and long-term. Moreover, this association was particularly pronounced among individuals with below-median incomes and those living in non-metropolitan areas.

Interpretation

The study’s findings indicate that hypertension-related avoidable hospitalizations elevate the risk of mortality, which aligns with previous research on the link between avoidable hospitalizations and mortality [30,31]. Since these hospitalizations adversely affect both short-term and long-term health outcomes, timely management of hypertension is essential for preventing hospitalization [32]. The study focused on hospitalizations occurring within the first year after the initial hypertension diagnosis. Despite this focus on the early post-diagnosis period, the results suggest that such events considerably influence later health outcomes, emphasizing the need for proactive management from the time of diagnosis. Previous research has demonstrated that early management of hypertension can lead to improved health outcomes [33-35]. Consistent with these findings, our results showed that patients in the adherence group (MPR ≥80%) during the first year after diagnosis experienced better short-term and long-term outcomes than those in the non-adherence group (MPR <80%). Additionally, we observed that the adherence group had a lower long-term incidence of hypertension-related avoidable hospitalizations compared to the non-adherence group (Supplementary Material 3). Korea’s primary care hypertension guidelines underscore the importance of improving the MPR to prevent health deterioration in patients with hypertension [36]. However, according to the Korean Hypertension Fact Sheet [6], the BP control rate among adults aged 65 and older (defined as systolic BP<140 mmHg and diastolic BP<90 mmHg) was only 59.4% between 2019 and 2021, indicating that nearly one-third of older hypertensive patients struggle to achieve adequate control. These findings highlight the need for targeted education to encourage active health management and appropriate treatment from the early stages of diagnosis.

This study also identified socioeconomic disparities in the relationship between hypertension-related avoidable hospitalizations and mortality. The results are consistent with previous studies that have highlighted income-related and region-related health disparities among patients with hypertension [13,37]. High-income patients typically have greater awareness of the importance of hypertension prevention and management, as well as better access to treatment [37]. In contrast, low-income patients exhibit lower health literacy [38], engage in unhealthy behaviors, and have a higher likelihood of comorbidities [39]. To reduce health disparities among older patients with hypertension, comprehensive BP management strategies and active education on healthy lifestyles are necessary. Regionally, Korea experiences significant differences in healthcare infrastructure between metropolitan and rural areas [13]. These infrastructural differences are closely linked to healthcare accessibility and contribute to regional disparities [40]. For example, data published by Korea’s National Medical Center in 2022 showed that nearly 90% of Seoul residents could access emergency care within 30 minutes, compared to only 44% and 41% of residents in rural areas such as Gangwon and Gyeongsang Provinces, respectively [41]. Similarly, 2020 data from the Ministry of Land, Infrastructure, and Transport revealed that the average distance to primary care facilities was 0.97 km in Seoul versus 11.05 km in Gangwon Province [42]. In our study, we found a higher incidence of hypertension-related avoidable hospitalizations among individuals living in non-metropolitan areas (Supplementary Material 4). These region-based findings suggest that higher hospitalization rates may reflect poorer primary care quality or limited healthcare accessibility [25]. Thus, the observed disparities could be attributed to differences in either the quality of or access to primary care across regions. However, since we did not include direct measures of primary care quality or accessibility, this interpretation remains indirect. Future studies should incorporate explicit indicators of primary care quality and accessibility to validate these findings further.

Limitations

This study had several limitations. First, it did not account for health behaviors such as smoking status, alcohol consumption, and physical activity, all of which may affect patients’ baseline health conditions. To partly address this limitation, adjustments were made for health-related and hypertension-related factors at the time of diagnosis. Second, given the nature of claims data, discrepancies may exist between recorded diagnostic codes and the actual diagnoses made by healthcare providers. To minimize such discrepancies, we included only patients who had at least 4 outpatient visits with hypertension as the primary diagnosis and who were prescribed antihypertensive medications, thereby reducing potential errors related to diagnostic code mismatches. Third, the time-dependent nature of the variable of interest raises the possibility of immortal time bias. We addressed this bias by employing the landmark method, an effective technique for its control. Fourth, complications arising during the follow-up period were not captured. Patients who experience a hypertension-related avoidable hospitalization are at higher risk for subsequent complications, particularly cardiovascular and cerebrovascular diseases, which may affect mortality. Although baseline cardiovascular and cerebrovascular diseases were adjusted for in our analysis, future studies should include new complications over a longer follow-up period. Fifth, patients hospitalized for hypertension-related avoidable events may have had a greater disease burden, reflecting more severe hypertension than those not hospitalized. To mitigate this, we adjusted for the CCI, ischemic heart disease, and cerebrovascular disease, and we conducted a sensitivity analysis using PSM to reduce selection bias. Despite these measures, residual confounding related to disease severity may persist.

Despite these limitations, this study is significant because it identifies a clear association between hypertension-related avoidable hospitalization and adverse short-term and long-term health outcomes in older patients with hypertension.

In conclusion, hypertension-related avoidable hospitalizations in older patients were associated with an increased risk of both short-term and long-term all-cause mortality. These findings underscore the importance of reducing such hospitalizations through timely and effective hypertension management. Although avoidable hospitalizations are generally considered indicators of primary care quality or limited healthcare accessibility, our study did not directly assess these factors. Therefore, any conclusions regarding primary care quality or accessibility should be made with caution. Nonetheless, our results suggest that enhancing primary care quality and improving healthcare accessibility may help reduce avoidable hospitalizations and ultimately improve outcomes in older adults with hypertension. Future research should incorporate explicit measures of primary care access, quality, and continuity to further validate these associations.